Patent Document 1 discloses a navigation device that implements high-speed drawing of a map using one video memory alone. The video memory of the navigation device in Patent Document 1 (VRAM 28 in FIG. 2 of Patent Document 1) has a size capable of storing a map image larger than a map image to be displayed on a screen. Then, a storage region of the video memory (VRAM 28 in FIG. 2 of Patent Document 1) is divided into r×s (such as 2×2) sub-regions. A bitmap map image generated based on map data read from a CD-ROM is stored in each sub-region.
A navigation control unit (indicated by reference numeral 23 in FIG. 2 of Patent Document 1) of the navigation device in Patent Document 1 receives vehicle current position data from a vehicle position measurement unit (indicated by reference numeral 21 in FIG. 2 of Patent Document 1), and checks whether or not a vehicle has reached a rewrite boundary line, and supplies the vehicle current position data to a window position control unit (indicated by reference numeral 26 in FIG. 2 of Patent Document 1). The window position control unit 26 calculates the position (window position) of an image to be extracted from the VRAM (indicated by reference numeral 28 in FIG. 2 of Patent Document 1), based on the received vehicle current position data, and supplies the calculated image position to an image extraction unit (indicated by reference numeral 29 in FIG. 2 of Patent Document 1). The image extraction unit (indicated by reference numeral 29 in FIG. 2 of Patent Document 1) reads the bitmap image of one screen from the VRAM (indicated by reference numeral 28 in FIG. 2 of Patent Document 1), based on the window position, and supplies the read bitmap image to a display unit (indicated by reference numeral 30 in FIG. 2 of Patent Document 1) to display the read bitmap image on the display unit. When the vehicle has reached the rewrite boundary line, the navigation control unit (indicated by reference numeral 23 in FIG. 2 of Patent Document 1) supplies an instruction of reading an adjacent map image to a map data reading control unit (indicated by reference numeral 24 in FIG. 2 of Patent Document 1), as shown in FIG. 4 of Patent Document 1. Then, the navigation control unit instructs a display control unit (indicated by reference numeral 27 in FIG. 2 of Patent Document 1) to rewrite the VRAM (indicated by reference numeral 28 in FIG. 2 of Patent Document 1). In this manner, whenever the vehicle moves, above-mentioned processes are repeated. A map image is scroll-displayed on a display screen (screen).
[Patent Document 1]
    JP Patent Kokai Publication No. JP-A-8-75480